Workflow adhoc triggering

DeveloperCYT

Hi, we have workflow enabled now and I was looking at it for Invoice approval process. Some days we have no invoices, and then other days we may get multiple invoices arriving.
From what I can see Workflow needs a schedule to run (daily, weekly, monthly etc), does that mean we can't trigger a workflow when we want to start an invoice approval flow, but have to wait for its scheduled start?